fix: separate session title from branch

[njbrake]

Description

• split the web session wizard into separate session title and branch fields instead of reusing one value for both
• keep branch creation tied to the branch field while making sidebar/breadcrumb labels show the session title for single-session workspaces
• stop the rename API from rewriting stored worktree branch metadata when only the session title changed
• closes Renaming session doesn't rename branch and pushes to a random named branch by default #931

Verification:

• cd web && npm run build
• cargo check --features serve
• cargo test --features serve --lib "server::api::sessions::tests::rename_updates_title_without_changing_worktree_branch" -- --exact
• cargo test --features serve --lib "server::api::sessions::tests::session_response_branch_from_worktree" -- --exact

PR Type

• New Feature
• Bug Fix
• Refactor
• Documentation
• Infrastructure / CI

Checklist

• I understand the code I am submitting
• New and existing tests pass
• Documentation was updated where necessary
• For UI changes: included screenshot or recording

AI Usage

• No AI was used
• AI was used for drafting/refactoring
• This is fully AI-generated

AI Model/Tool used:
Codex (GPT-5)

Any Additional AI Details you would like to share:
Implemented the web title/branch split and rename metadata fix directly from issue triage for #931.

• I am an AI Agent filling out this form (check box if true)

[Seluj78]

Ok so testing this PR locally, I've got a small gripe with it. When clicking on either of the Auto Generated fields I would expect a text box where I can override, but instead I get sent to the session wizard where I need to go to advanced settings, which isn't great UX in my opinion :)