Replace ProminentButtonModifier with standard button rows and styles

[bradleymackey]

Applies the button convention from #551 to the screens outside
Views/Backup, and deletes the modifier that made the old treatment
possible.

The problem

ProminentButtonModifier reimplemented .borderedProminent by hand:
.font(.headline) + a forced .foregroundStyle(.white) + padding(16/12)

• .background(color) + RoundedRectangle(cornerRadius: 12). It applied
  .buttonStyle(.borderless) inside its own body, so the wrapped button
  never received a pressed state, and the hardcoded white label had no
  contrast guarantee against the tint it was placed on.

Six of its nine call sites also placed the button inside a Section
footer as a padded, centred pill — the exact pattern #551 called out and
removed from the backup screens.

What changed

Screen	Change
SettingsDangerView	"Delete All Data" becomes a red row in its own section; the error message becomes that section's footer
VaultTagDetailView	"Delete Tag" becomes a red row
VaultItemDetailView	"Unlock" and "Dismiss" move out of section footers into their own rows
EncryptedItemDetailView	"Decrypt" moves out of the footer into its own section
VaultDetailEncryptionEditView	"Encrypt" and "Remove Encryption" become plain rows
OTPCodeDetailView	"Delete" moves out of the editing-actions footer into its own red section
SecureNoteDetailView	"Delete" likewise
VaultAutofillConfigurationView	"Continue" is a genuine standalone CTA outside any Form, so it becomes .buttonStyle(.borderedProminent) + .controlSize(.large)

Destructive emphasis is preserved the way #551 preserved it: a red
FormRow glyph tile plus red row text, matching BackupRestoreView's
Import & Override. Primary actions use the accent colour the same way
BackupKeyDecryptorView does.

The ProgressView in each loading: branch loses its .tint(.white),
which only existed because the spinner sat on a filled pill.

Toolbar Cancel buttons keep their explicit .foregroundStyle(.red).
Button(role: .cancel) does not render red in a toolbar, so dropping the
tint would have been a visual regression rather than a standardization.

Liquid Glass does not survive snapshot rendering

"Continue" was first written as .buttonStyle(.glassProminent). That
rendered VaultAutofillConfigurationView as a completely blank image
— not just the button, the whole hierarchy — dropping the reference from
151KB to 62KB.

Liquid Glass samples a backdrop through the render server, and the
.image snapshot strategy rasterises off-screen, so the effect resolves
to nothing and takes the rest of the frame with it.
Snapshotting.image(drawHierarchyInKeyWindow:) exists as a possible
escape hatch but changes the rendering path for every test, so it is not
something to adopt as a side effect of this PR.

.borderedProminent is used instead. On iOS 26 the system already draws
it as a capsule, which is the shape the hand-rolled 12pt rounded
rectangle was imitating.

Verification

Local, iPhone 18 Pro Max / iOS 27.0:

• xcodebuild build-for-testing — TEST BUILD SUCCEEDED
• Affected suites pass: OTPCodeDetailViewSnapshotTests,
  SecureNoteDetailViewSnapshotTests,
  VaultAutofillConfigurationViewSnapshotTests
• 43 snapshots re-recorded (18 OTP detail, 24 secure note, 1 autofill)
  and visually reviewed
• make format + make lint — clean

SettingsDangerView, VaultTagDetailView, EncryptedItemDetailView and
VaultDetailEncryptionEditView have view-model tests but no snapshot
coverage, so there was nothing to re-record for them.

⚠️ Automatic CI is still disabled (#548), so this is local verification only.

🤖 Generated with Claude Code