web UI: Rename opens blank for untitled sessions and mislabels empty as "clear" #1078

A session with no explicit title displays a derived name in the list (e.g. shell), but the Rename sheet opens with an empty field and the placeholder leave empty to clear. The user sees a name on screen, opens Rename to change it, and is shown a blank box that claims emptiness will "clear" something that already looks empty.

Repro

1. Create a session without a title — the list shows the derived name, e.g. shell.
2. Open Rename on that row.
3. The title field is blank; the placeholder reads leave empty to clear.

Cause

openRenameDialog() (crates/daemon/assets/index.html ~line 14537 @ 4c1459b) prefills from the raw override:

renameTitleEl.value = s.title || "";

s.title is the user override, which is unset; the visible name comes from sessionDisplayTitle(s). Submitting empty omits title from the RPC so the daemon clears the override — correct behavior, badly explained.

Expected

The dialog should make the two-state model legible: there is a derived default, and an optional override on top of it.

• Placeholder shows the derived name the session would fall back to, e.g. shell (default), instead of leave empty to clear.
• Copy states the actual semantics: empty restores the default title, it does not blank the name.
• Where a user override is set, offer an explicit "Reset to default" action rather than making the user discover that clearing the field does it.

Acceptance criteria

• Opening Rename on an untitled session makes clear what the current displayed name is and where it comes from.
• The empty-field behavior is described as "restore default", not "clear".
• Resetting to the default title is reachable without guessing.

Found during a power-user UX audit of the TUI and web UI.