fix(web): new threads keep the viewed thread's model

[Gigioxx]

Since #6011 the project default model outranks the model carried from the thread being viewed. Every project gets a default seeded at creation (the first selectable provider's default), so in practice a new thread always reset to that model instead of keeping the one the user was working with. Runtime and interaction modes still carried; only the model was lost.

This flips the priority in resolveNewThreadModelSelectionOverride: the carried selection wins, and the project default only fills in when there is nothing to carry (fresh app, or the draft would carry from itself). Mobile is unaffected since its new-task flow has no viewed-thread carry.

Validation

• vp test run apps/web/src/lib/chatThreadActions.test.ts (10 tests)
• vp fmt --check and vp lint on the touched files
• vp run --filter @t3tools/web typecheck

Built with Claude Fable 5 in Claude Code.

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Note

Low Risk
Localized priority change in new-thread model resolution with updated unit tests; no auth, data, or API surface changes.

Overview
New-thread model selection no longer resets to the project default when the user was already working with a different model on the thread they were viewing.

resolveNewThreadModelSelectionOverride now prefers the carried composer/thread model over the project default (which is always seeded at project creation). The project default is only applied when there is nothing to carry—e.g. cold start or when the destination draft would carry from itself. Comments in useHandleNewThread are updated to match.

Tests are updated and extended to assert carried-over-wins, plus fallback to project default when carry is absent.

^{Reviewed by Cursor Bugbot for commit 759df78. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix resolveNewThreadModelSelectionOverride to prefer carried model over project default

• New threads now inherit the viewed thread's model, runtime mode, and interaction mode, with the carried model taking precedence over the project default.
• chatThreadActions.ts reverses the precedence in resolveNewThreadModelSelectionOverride: it returns the carried selection when present, and falls back to the project default only when there is nothing to carry (including when source and destination draft IDs match).
• chatThreadActions.test.ts updates tests to expect carried-selection precedence and adds a fallback-to-default test.
• Behavioral Change: resolveNewThreadModelSelectionOverride no longer lets the project default override a carried selection; consumers relying on the old order will see carried selections win.

^{Macroscope summarized 759df78.}

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[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — New threads will now favor the viewed thread’s model over the configured project default whenever both are available, changing the product’s effective default-selection behavior. The change is localized and tested, but that default-policy change warrants human review.

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