Allow renaming session tabs (custom tab names)

[computerbox124]

Summary

Session tabs are always named after the host's label (or "Local"). When several tabs connect to the same host — or when the user wants a task-oriented name like "db-migration" — they are indistinguishable. Add the ability for the user to rename a tab, with the custom name persisted in saved workspaces.

Proposed behavior

• Rename interaction: double-click the tab in the strip to edit inline, plus a "Rename" entry in the tab context menu. Enter commits, Escape cancels, empty/whitespace input falls back to the default name.
• Custom name takes precedence over auto-derived titles and is shown everywhere the title is used: tab strip, tooltips, close aria-labels, "Move to …" menu entries, drag chips.
• Cap name length; duplicate names are allowed.

Implementation notes

• Tab.title already exists in desktop/src/stores/tabs.ts and is rendered in TerminalArea.vue.
• Add a customTitle/lockedTitle flag on Tab so syncTabTitle (which re-derives the title whenever panes are closed, moved, or swapped) does not clobber a user-set name. Decide whether the name survives splits/merges or resets.
• Persistence: TabLayout.title is already saved/restored by saveCurrentLayout/restoreWorkspace in WorkspaceView.vue, so custom tab names survive workspace save/restore — but the customTitle flag must be persisted in TabLayout too, otherwise a restored tab loses its name on the first structural change.
• Precedence: user-given name > setPaneTitle-derived/auto-derived title.
• Optional follow-up (out of scope): per-pane rename. Note that pane titles are currently not persisted — paneTreeToLayout/layoutToPaneTree recompute them from host.label — so this would need a title field on PaneLayout.

Acceptance criteria

• User can rename a tab via double-click and context menu
• Custom name survives pane close/move/split within the tab
• Custom name (and its stickiness) persists through workspace save/restore
• Empty rename input reverts to the auto-derived name