fix: rename input pre-fills the session title and selects all text

[windli2018]

What

When renaming a session, the rename input now pre-fills the displayed title and selects all of it — so you can type a new name to replace it, or edit in place.

Why

Before, the input was pre-filled with session.name only. Sessions without an explicit name (most of them) have an empty session.name, so the input appeared blank — even though the sidebar displays a fallback title (first message / id). Also the pre-select ran via setTimeout(..., 0) before the input had mounted, so it often didn't select anything.

How

• startRename fills session.name || firstMessage.slice(0,50) || id.slice(0,12) (the same fallback the row title uses).
• Selection happens in a useEffect on renaming (after the input is mounted, via requestAnimationFrame).
• commitRename no-ops when the value equals the displayed fallback title — so the fallback isn't accidentally persisted as a real name.

Validation

• Rename on a session without a custom name → input shows the first-message title, fully selected.
• Type to replace, or edit in place; unchanged submit doesn't persist the fallback as a name.