Le drag-and-drop HTML5 natif est peu fiable dans les webviews WebKit
qui font tourner l'app Tauri : dragstart ne se déclenche souvent pas.
On pilote le réordonnancement avec des événements souris à la place.

🪄 Commit via GitWand

Addresses correctness defects in the RepoTabStrip.vue pointer-drag
rewrite found in PR #121's review:

- Hit-test now compares the dragged tab's own shifted center (offset
  from its drag-start position) against the other tabs' centers,
  instead of the raw cursor position — a press near a tab's edge no
  longer overshoots into a reorder from a few pixels of hand wobble.
- Only the left mouse button can end a drag; a right/middle-click
  released mid-drag (e.g. after middle-click closed another tab) is
  ignored instead of committing a stale reorder.
- Releasing well above/below the strip, pressing Escape, or losing the
  mouseup to a window blur (alt-tab, native dialog) cancels the drag
  instead of always committing wherever the cursor lands.
- Tab centers are re-snapshotted on strip scroll, and a tab
  opening/closing mid-drag now aborts the drag instead of committing
  against a stale position/index snapshot.
- `didDrag` resets on a deferred tick rather than only via the
  trailing click, so it can't get stuck and swallow the next keyboard
  tab activation when the mouseup lands off the dragged tab.
- Dropped the unconditional `preventDefault()` on tab mousedown, which
  was suppressing click-to-focus on the tablist (text selection is
  already blocked by `user-select: none` in CSS).
- Removed the dead `.repo-tab__caret` branch from the drag-start guard
  (the caret already stops propagation on its own mousedown).

Adds regression coverage for the wobble/reorder, wrong-button, and
cancel-outside/blur cases.
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Bare Ctrl+ArrowLeft/Right collides with macOS's default Mission
Control "switch Space" shortcut, which the OS intercepts before the
keydown ever reaches any app — a browser included, so this isn't
Tauri-specific. Cmd+Arrow isn't bound by default, but third-party
window-manager utilities and user remaps commonly claim bare
Ctrl/Cmd+Arrow too.

Require Shift alongside Ctrl/Cmd (Ctrl/Cmd+Shift+ArrowLeft/Right) to
stay clear of every OS-level and third-party shortcut we know of for
this combo, the same reasoning VS Code's editor-move commands use a
Shift-modified combo rather than a bare arrow. Updates the
aria-keyshortcuts hint and adds a regression test confirming a bare
Ctrl+Arrow (no Shift) no longer triggers a reorder.
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