feat(extension): open the popup from the inline picker's more-logins row

[rendyuwu]

What

The inline picker's N more logins on <domain> footer is now a row that opens
the Subclave popup through the service worker. Where the popup cannot open,
the footer turns into text naming the browser's Extensions menu and the
fill-login shortcut.

Why

Closes #20.

Following the old footer meant leaving the field for the browser's toolbar,
where the icon usually sits inside the Extensions menu until it is pinned, and
the footer never named the Ctrl+Shift+L command. The exact-host rule is
unchanged: the picker still lists this host's entries only, the row fills
nothing and releases nothing, and the wider matches show only in the popup.

How

• New inline request inline-open-popup (extension/src/lib/messages.ts),
  accepted from the top-frame content script only, like every inline request.
  handleInline runs openPopup and nothing else, and reads neither the tab
  nor the URL. It answers { type: "popup", ok: true }, or ok: false with
  the fill-login shortcut from chrome.commands.getAll() ("" when
  unbound).
• openPopup in extension/src/background.ts is now the only caller of
  chrome.action.openPopup; a missing member or a rejection reads as false.
  runFillCommand uses it too and keeps its newest-lastUsedAt fallback.
• showReady (extension/src/content/inline.ts) adds the row last, after the
  entry rows and the generate and update rows. On success the picker closes.
  On failure popupFallback keeps the shortcut and the same picker is drawn
  again with the text in place of the row. close() leaves popupFallback
  set, so the rest of that page shows the text and never asks again.

Decisions on the open points in #20:

• Wording: the row reads N more logins on <domain> / Open in Subclave popup.
  The fallback reads N more logins on <domain>: open Subclave from the browser's Extensions menu, or press <shortcut>, without the , or press
  part when the command is unbound.
• Guards: the row needs event.isTrusted only, like Unlock and
  Pair with Subclave. It fills nothing and releases nothing, so a
  clickjacked click on it can only open the popup, where a fill still takes a
  click in browser-owned UI.
• Firefox before 149 and Chrome before 127: no version check. The row shows,
  and its first failing click on a page turns it into the text for the rest of
  that page.
• The guard refusal text in refuse is unchanged and gets no row. Under
  guard 2 the picker is inert behind a modal, so a row there could not be
  clicked.

Docs:

• SECURITY.md: the row, what it can and cannot do, and its guard rule.
• ARCHITECTURE.md: the new request in Browser fill > Inline.
• KNOWN-LIMITS.md: the browsers where the row falls back to text.
• README.md: new "Fill a login" section covering the picker, the
  more-logins row, Ctrl+Shift+L / Cmd+Shift+L, and where to set the
  shortcut when another extension holds it.
• CHANGELOG.md: an ## [Unreleased] entry. scripts/release-notes.mjs
  reads versioned headings only, so the 0.1.1 notes are unchanged.

Testing

• pnpm --filter subclave-extension test: 84/84 across chrome and
  chrome-inject.

• Headless Chromium resolves the real openPopup without showing a popup and
  leaves fill-login unbound, so the new tests stub chrome.action.openPopup
  and chrome.commands.getAll in the service worker.

  • "the more-logins row opens the popup by pointer and by keyboard, and
    ignores an untrusted click": one other match in no-fields mode, by
    pointer; two in ok mode, reached past the entry rows with ArrowDown
    and Enter. An untrusted click opens nothing and shows no refusal, and no
    get-credential reaches the app.
  • "where the popup cannot open, the more-logins footer names the Extensions
    menu and the shortcut": a rejecting openPopup turns the row into the text
    ending in , or press Ctrl+Shift+L. Reopening the picker on the same page
    shows the text at once without calling openPopup again. With openPopup
    deleted and the command unbound, a fresh page shows the text without
    , or press.

• Removed "the footer counts the logins left for the toolbar button", which
  checked the old footer text. "opens on a field click, on the icon and on
  ArrowDown, for this host only" now expects FAKE_ENTRIES.length + 1
  options: in ok mode the fake app reports two other matches, so the picker
  carries the row.

• Each of these breaks, made one at a time and then reverted, fails one of the
  new tests:

  • no close() after the popup opened: the picker stays open;
  • the row guarded: true: the untrusted click is refused;
  • popupFallback never set, or reset in close(): the row stays, or comes
    back on reopen;
  • the shortcut looked up as "fill": the text lacks , or press Ctrl+Shift+L;
  • a missing or rejecting openPopup read as success: the picker closes and
    no text shows;
  • inline-open-popup missing from INLINE_REQUEST_TYPES: openPopup never
    runs.

• pnpm run lint:imports, pnpm run typecheck:scripts,
  pnpm run format:check, pnpm run verify, pnpm exec tsc --noEmit and
  pnpm run typecheck:extension are clean. Both extension builds pass
  (content.js is 18 KB of the 30 KB budget), lint:webext reports 0 errors,
  and zip packs both zips.

• pnpm exec tsc --noEmit clean

• Manual smoke-test of the affected feature

• (If you touched src-tauri/) cargo check clean

• (If UI) tested in pnpm tauri:dev (isolated dev data dir)

No changes under src-tauri/ and no app UI change.

Not tested yet:

• The real popup opening from the row in a headed Chrome 127 or later, with a
  github.com entry and a login field on gist.github.com.
• Any Firefox, including whether Firefox before 149 accepts the call from a
  click relayed by the content script.

Screenshots / GIFs

None. The change is in the extension's inline picker, and it has not been run
in a headed browser yet.

Notes for reviewer

• popupFallback lives per page. On a browser that can never open the popup,
  each new page shows the row until its first click there. KNOWN-LIMITS.md
  records this, and it changes once the service worker can tell before the
  first click that the popup cannot open.