feat(app): show a grip on the resizable edge of the navigation

[slyk]

Change description

Part of discussion #431 (item 2.1).

The navigation panel can be resized, but its handle is invisible until the pointer is exactly over it, so most users never find it. This PR adds a small, always visible grip to that edge.

• The handle moves into a new panel-resize-handle.vue component. private-view.vue keeps its existing resize logic and only uses the component (the module-nav-resize-handle class is kept for custom CSS).
• The grip:
  • A 6×48px tab with a column of dots, centred vertically.
  • It sits on the panel border: square on that side, fully rounded on the other.
  • It is outlined in the panel background colour, so it stays visible over the section headers that share its colour.
  • All colours come from theme variables, for light and dark themes.
• Hover: the grip and the line light up in the primary colour after the same short delay as the existing line, so they do not blink when the pointer just passes by. While dragging they light up at once.
• The hover line is thinner (2px instead of 4px), since the grip already shows where to drag.
• The hit area is wider: 12px instead of 4px, 8px inside the panel and 4px over the content.
• Resizing behaves as before.
• The component also supports a handle on the left edge of a panel, ready for a resizable sidebar (a later PR from UI improvements (sidebars, primary color, login animation) #431).

Type of change

• Bug fix (fixes an issue)
• New feature (adds functionality)

Related issues

Discussion #431

Checklists

Development

• Lint rules pass locally
• Application changes have been tested thoroughly
• Automated tests covering modified code pass

Security

• Security impact of change has been considered: none, UI only.
• Code follows company security practices and guidelines

Network

• Changes to network configurations have been reviewed (not applicable)
• Any newly exposed public endpoints or data have gone through security review (not applicable)

Code review

• Pull request has a descriptive title and context useful to a reviewer. Screenshots or screencasts are attached as
  necessary
• reviewers assigned
• Pull request linked to task tracker where applicable