[FEATURE] Desktop: right-click context menu on transcript text with AI actions (explain, spawn new session, quick answer)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In the desktop app, acting on part of a response requires manual copy/paste and re-typing prompts. Common cases:

• I read a paragraph in a long response and want it explained in more detail, or want more context — today I have to copy it, start a new session, and write a prompt around it.
• I want to execute an instruction/step from a response in a separate thread (sometimes with the same model and effort as the current thread, sometimes with a different one) without derailing the current conversation.
• I hit an unfamiliar word or phrase and just want a one-line definition. Sending it as a full prompt to the main model is slow and heavyweight for a "what does this mean?" lookup.

The transcript text is inert today — there is no way to attach actions to a selection or paragraph.

Proposed Solution

Add a right-click context menu on transcript text (a selected word/phrase or the paragraph under the cursor) with configurable AI actions:

1. Explain this in more detail — starts a new session seeded with the selected text.
2. Give me context — same, but asks for background/context.
3. Execute this in a new thread (same model + effort) — runs the selected instruction in a new session inheriting the current thread's model and effort settings.
4. Execute this in a new thread (choose model + effort) — opens a small picker dialog first.
5. What does this mean? — for a highlighted word/phrase: an instant inline answer from a fast, cheap model (e.g., Haiku) shown in a tooltip/popover, without a full prompt round-trip in the main conversation.

All five share one underlying capability: a context menu on transcript text whose actions receive the selection as input. A plugin/config surface for user-defined menu actions would make this extensible.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Other

Use Case Example

Example scenario:

1. Claude finishes a long analysis of my retirement-planning project; one paragraph proposes a data migration step.
2. I right-click that paragraph → "Execute this in a new thread (same model + effort)" — the migration runs in its own session while I keep discussing the analysis in the current one.
3. Later, the response uses the term "idempotent". I highlight it, right-click → "What does this mean?" and get a one-line definition from a fast model in a popover, without interrupting the session or paying for a full-context prompt.

Additional Context

_No response_