feat(app): add full session option to web fork dialog

[HopelessLoop]

Issue for this PR

Closes #37016

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Web fork dialog only supports forking up to a selected message — there is no way to fork the entire conversation. CLI and TUI both support full-session fork. This adds a "Full session" option as the first item in the web fork dialog list. Selecting it calls session.fork() without messageID, copying all messages. Behavior matches the existing TUI implementation in DialogForkFromTimeline.

How did you verify your code works?

Tested locally: opened fork dialog, selected "Full session", verified new session contains all messages from the original. Existing message-based fork path unchanged.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Web fork full doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.