Add REUSE/SPDX compliance headers to all files

Dolphin was missing copyright and license information on ~750 files,
which would cause the KDE CI job `gitlab-templates/reuse-lint.yml` to fail.
This commit makes the project fully compliant with REUSE Specification 3.3.

REUSE.toml is used for file categories that cannot carry inline comments
or are managed externally:
- po/**: covered with precedence=aggregate so the bulk declaration
  combines with individual translators' existing inline headers
- **/*.kcfg, **/*.kcfgc: KConfigXT schema files, no comment syntax
- doc/*.png: documentation screenshots
- src/icons/*.png: application icons (LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL)
- src/**/*.{qrc,rc,desktop,xml,json,knsrc,upd,in}: resource and
  metadata files without usable comment syntax
- .gitignore, .git-blame-ignore-revs: infrastructure, CC0-1.0
- logo.png: application artwork (LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL)

Inline SPDX headers are added to all source files that support comments
(C++, QML, CMake, shell scripts, Python). A small number of remaining
binary or single-purpose files retain individual .license sidecars.

License inference heuristic (applied per file, in order):
1. Existing inline SPDX-License-Identifier — reused as-is
2. Most common license among sibling files in the same directory
   that already carry SPDX info, excluding infrastructure filenames
   (.gitignore etc.) and requiring at least two qualifying neighbors
   to prevent a single CC0 CI config from pulling source files along
3. Default: GPL-2.0-or-later

Copyright years and authors are sourced from `git log --follow` per
file. Automation accounts (l10n daemon [email protected], Weblate) are
excluded from author lists. If more than 5 authors are found only a
generic KDE Contributors was used.

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