fix(web): files panel right-click actions and drags use the folder's real path

[dotrahat]

In a files panel with several folders, right-clicking a file or dragging it into the composer used the folder name shown in the tree as if it were a path in the workspace folder. For a listed folder outside the workspace folder, a nested listed folder, or an isolated run's worktree, the menu actions and the mention pointed at the wrong file. Details are in #335.

Fix

The right-click menu and drag-to-composer now resolve each entry through its real folder, the way left-click already does. A new fileTreeEntryTarget helper in filePath.ts turns a tree entry into its folder and path, and both use it through FileBrowserPanel. A top-level folder entry such as notes now means that folder itself.

Mentions from a multi-folder tree use the file's full path, as @ search already does for files in another folder. So Copy mention on api/README.md now gives /home/<user>/dev/scope-test-four/api/README.md rather than api/README.md. Single-folder projects are unchanged and still get relative mentions.

Testing

• New unit tests cover a folder outside the workspace folder, a nested folder, a top-level folder entry and the single-folder case, plus a drag mention resolved by the host. All 129 tests in components/files and composerMentionDrag.test.ts pass, the web typecheck is clean, and lint reports the same warnings on the changed files as multi-repo-workspaces. I didn't run the full suite.
• Tested in AppImages built from this branch, with the Files panel: right-click actions use the folder's display name, not its real path #335 example project. The right-click rows were tested on the first version of the commit; the drag rows and a re-check of Reveal in Files and Copy mention ran on the final one, after the right-click handler moved to the shared helper.

Action	Result
ideas.md (in ../notes) → Reveal in Files	Opens ~/dev/notes
ideas.md → Copy mention	[ideas.md](/home/<user>/dev/notes/ideas.md)
ideas.md → Open with → VS Code	Opens the real file
ideas.md → Add to chat, "What is the first line of this file?"	The agent reads ~/dev/notes/ideas.md directly
plan.md (in notes/2026) → Reveal in Files / Copy mention	Opens ~/dev/scope-test-four/notes/2026 / full path
The notes folder → Reveal in Files	Opens ~/dev, the folder that contains it
api/README.md → Reveal in Files	Opens ~/dev/scope-test-four/api, as before
Drag ideas.md, plan.md or the notes folder into the composer	The mention has the full path
WORKTREE.md in an isolated run → Reveal in Files / Copy mention	Opens the worktree's api folder / full worktree path
A file in a single-folder project → Copy mention, drag	Relative path, as before

Fixes #335

Model: Claude Opus 5.5, harness: Claude Code.

[LoganRupe]

Tested against multi-repo-workspaces (feee30eed) with the #335 layout. Right-click ideas.md (from ../notes), Add to chat, then click the chip. Before, it fails with "Failed to read workspace file 'notes/ideas.md'". With this branch it opens ~/dev/notes/ideas.md. The 129 tests pass locally too. LGTM.

compare.mp4