fix: resolve user templates in remote PPTX generation

[ShotaroKataoka]

Summary

Fixes #206 — custom templates uploaded via the Web UI were silently ignored during PPTX generation on AWS (Layer 3/4), falling back to blank-dark.pptx with no warning.

Root cause

Template resolution in _prepare_workspace() (servers/remote/tools/generate.py) only matched against storage.list_templates() (builtin templates). User templates were never searched, and the unresolved name silently fell back to the default.

Changes

• Resolve user templates first via get_user_template_metadata / download_user_template — same resolution order as analyze_template
• Raise ValueError (with the available template list, same format as analyze_template) when an explicitly referenced template cannot be resolved — no more silent wrong-design fallback
• Remove the deck.get("templateS3Key") fallback: never set by production code (create_deck), it existed only in the test fixture
• Keep the blank-dark.pptx default when no template is specified (unchanged behavior)

_prepare_workspace is shared by generate_pptx and the compose/preview verify pass, so one fix covers both paths.

Testing

• 3 new unit tests: user template resolution, unresolvable name → error, empty template → default (regression)
• Test fixture updated to match production deck records (dropped templateS3Key)
• make lint / make test pass (758 passed, 5 skipped)
• Verified end-to-end on AWS: deck referencing an uploaded user template now builds with that template (52 layouts + media present in the generated PPTX; fonts extracted from the user template)

Out of scope

• Local server (servers/local) — separate logic, unaffected
• Per-slide background overrides written by composers can still hide a template's background image; that is spec-driven behavior, not a resolution bug (can be tracked separately)