fix(sources): report files skipped during import

[annieyii]

Fixes #675.

Problem

Importing sources dropped files silently. Every gate in the import loop
discarded the file with a bare continue, a failed copyFile only reached
console.error, and handleImport wrapped the call in try/finally with
no catch, so a thrown import became an unhandled rejection that never
reached the UI.

The file dialog also offers Images, Media, Code and jsonl/tsv/ndjson
filters that INGESTABLE_SOURCE_EXTENSIONS does not accept, so those files
could be selected, confirmed, and discarded without a word.

Change

importSourceFiles and importSourceFolder now return
{ imported, skipped }, where each skipped entry carries a reason:

Reason	Cause
unsupported-type	Not in INGESTABLE_SOURCE_EXTENSIONS
excluded	excludeExtensions / excludeDirs / excludeGlobs / hidden file
too-large	Over maxFileSizeMb, with the actual size as detail
unreadable	getFileSize threw
copy-failed	copyFile threw
sensitive-config	Withheld by isSensitiveConfigSourceFile

The sources view lists them with the same per-item pattern URL import already
uses, and both handlers now catch a thrown import instead of logging it.

Exclusions are checked before the ingestable-type test, so a hidden file that
is otherwise a supported type reports excluded rather than the misleading
unsupported-type.

Notes

• Scope is limited to reporting. The three adjacent problems listed in Source imports drop files silently: no error, no indication which files were skipped #675
  (silent no-op when no LLM is configured, the size-limit field being
  disabled while the limit still applies, and drag-and-drop) are left alone.
• Does not touch drag-and-drop, so it should not conflict with feat(sources): add desktop drag-and-drop imports #661. That
  path will need the same reporting once it lands.
• Related to import a folder using deb version do nothing #572. I would not claim it closes that report: the symptom there
  has at least six possible causes that all look identical today, which is
  the point of this change. After it, the reporter can re-run and get a
  reason. The swallowing code is unchanged between v0.6.3 (the version in
  that report) and main.

Testing

npm run test:mocks — 127 files, 1832 tests pass. npm run typecheck clean.

10 new tests cover each skip reason for both entry points plus the display
decision, written test-first. Five existing tests were updated for the new
return shape. New i18n keys are added to all four locales so
i18n-parity.test.ts stays green.

Also verified by hand in tauri dev: importing a 142 MB PDF against the
default 100 MB limit now reports too-large with the file size instead of
doing nothing.

🤖 Generated with Claude Code

[annieyii]

This landed on main as fd9c953. Thanks!

Shipped in v0.6.10. Closing as superseded by the direct commit.

Ref: #675