Add smart-fit as default photo fit mode

[cbaucom]

Summary

• Adds smart-fit as the default global fit mode (themes included). At plan time it resolves per slide from media vs canvas aspect ratio: orientation/shape mismatches → blur-fill, compatible orientations → cover, unknown dimensions → contain.
• Extracts intrinsic width/height when media is loaded (image bitmap + Mediabunny video track) so the planner can make fit decisions.
• Passes project aspectRatio into plan(); RenderPlan entries still carry only concrete fit modes (cover | contain | blur-fill). Per-slide overrides unchanged.

Code review

Score: 4/5 — APPROVE

Blocking findings

None.

Non-blocking findings

• media-loader.ts — video files open two separate Mediabunny Input instances (duration + dimensions). Could merge into one parse in a follow-up.
• planner.ts — aspectRatio is a 7th positional parameter; consider an options bag if more plan inputs accumulate.
• No unit test for mediaMetadata width/height fallback path (slide lacks dims, metadata has them).
• createImageBitmap is not stubbed in import tests; dimension extraction silently no-ops on invalid blobs in test env (acceptable for now).

Strengths

• Fit decisions stay in pure modules (smartFit.ts + planner); composition unchanged.
• Deterministic resolution from slide dimensions + canvas aspect ratio.
• Good table-driven coverage in smartFit.test.ts plus planner integration tests for the key cases.
• Existing projects with saved fitMode: 'cover' are unaffected.

Test plan

• pnpm test
• pnpm lint
• pnpm build
• Manual: 9:16 canvas + 16:9 photo → full image visible with blur-fill (Smart fit global)
• Manual: switch global to Cover (crop) → photo crops again
• Manual: refresh folder after upgrade so slides pick up dimensions
• Manual: per-slide fit override still wins over global smart-fit

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