[FedyaLight]

Every .xml path was imported as a timeline. Canon writes clip metadata as
XML beside each clip (CONTENTS/CLIPS001/*.XML), so selecting clips together
with their XML files stopped the whole run:

• one XML: Timeline import: No FCP 7 XML sequence was found in A001C001_….XML..
• two or more: Add one timeline at a time.

Folder imports already skipped these files; only paths added one by one, for
example by selecting every file of a clip in the Add Media dialog, reached
the timeline importer.

Changes:

• An XML file is a timeline only when its root element is FCP 7 xmeml or
  fcpxml. Recognizing it reads at most 16 KiB of the file. A UTF-16 file is
  decoded to find its root: a UTF-16 project stays a timeline, so its import
  reports that it cannot be read instead of the run silently continuing
  without it. Other files that cannot be read or parsed still count as
  timelines for the same reason.
• The pipeline's path expansion uses the same check as the CLI and the app,
  instead of its own extension test.
• The app recognizes timeline inputs once, when they are added, and keeps
  them in its session state. Rendering the error window's "Locate media"
  button and the path fixer's "Save Fixed Copy" no longer read any file.
• The timeline import error no longer ends with a doubled period.

Reproduced with the reporter's layout from #16 (Canon C70 MXF and Canon MP4
with their XML and MIF files, plus a recorder folder), each file selected
individually: 0.3.2 stops with Add one timeline at a time.; this branch
(together with the #16 and #17 branches) finds both pairs at the correct
offsets.

Tests:

• camera_metadata_xml_beside_media_is_not_a_timeline fails on main.
• xml_inputs_are_explicit_not_silent now adds the XML as a path and checks
  the message; it fails on main on the doubled period.
• timeline_documents_are_recognized_by_root_element keeps Premiere, Resolve
  and FCPXML exports (declaration, DOCTYPE, comments, upper-case root) and
  UTF-16 projects with and without a byte order mark recognized, and Canon and
  Sony camera XML not.
• timelines_in_other_encodings_are_reported_not_skipped: a UTF-16 project
  added beside media ends in "Cannot read timeline", not a run without it.

The encoding tests were checked by removing the UTF-16 handling: both
fail.

Validation:

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace: 376 passed, 1 ignored
• Windows was not run locally.