feat(browser): add window-level tabs with a docked tab strip

[JoeJoeflyn]

Closes #108 (pane-specific items deferred until #83 lands).

What changed

• New TabsModel (src/ui/tabs.rs): each tab owns an independent live BrowserView in a shared gtk::Stack, so switching is instant and inactive tabs keep their locations, histories, and selections.
• Tab strip docks on top of the file viewer (inside the content pane), hidden until a second tab exists; a + header button is always visible. Styling reuses the column-header visual language with semantic @theme_* colors.
• New tabs open at the current location (Home only as fallback), matching Nautilus/Dolphin.
• Shortcuts: Ctrl+Shift+T new tab, Ctrl+W close, Ctrl+Tab/Ctrl+Shift+Tab and Ctrl+PgDn/Ctrl+PgUp switch, Ctrl+1–Ctrl+9 direct select, Ctrl+S chord group (T/N/P/X/digits) per the issue's proposal, Ctrl+Shift+PgUp/PgDn reorder.
• Pointer: click to switch, close button, middle-click close, drag to reorder, drop files onto a tab to transfer there.
• Folder context menu gains "Open in New Tab" (directories, not Trash).
• Shared chrome routed to the active tab on every use: sidebar places, search results, settings toggles, terminal/refresh actions, rename/location click-away, appearance menu, keyboard navigation. The header breadcrumb swaps to the active tab's location widget on switch.
• Window destroy clears observers for every tab's browser.

Manual steps

1. cargo run, press Ctrl+Shift+T or click the header + → tab strip appears above the file list with the current folder's name.
2. Switch with Ctrl+Tab, Ctrl+2, or by clicking tabs; breadcrumb, sidebar highlight, and keyboard focus follow the active tab.
3. Right-click a folder → "Open in New Tab" → opens in the background's new active tab.
4. Drag a tab to reorder; middle-click or Ctrl+W to close; the last tab refuses to close.
5. Change a settings toggle (e.g. folder peeking) → applies to every tab.
6. Drop a file onto another tab's label → transfers to that tab's location.

Expected result

Multiple independent workspaces in one window, each keeping its own history and selection, with no observer leaks on close.

Skipped for v1: session persistence (out of scope per #108) and detach-tab-to-window.

N/A — screenshots: tab strip styling reuses existing column-header CSS; behavior is interactive (drag/reorder/drop) and not capturable in a still.