…thersdr#3164)

## Summary

Closes [RFC aethersdr#3076](aethersdr#3076)
by adding a share-friendly file format for AetherSDR themes plus the
import / export / drag-and-drop UI that operators need to actually swap
themes around.

**Depends on aethersdr#3160** — currently targets `auto/theme-pr4` so the diff
shows only Phase 6's delta. Will retarget to main once aethersdr#3160 lands.

## ThemeManager additions

- `exportThemeToFile(themeName, filePath, *err)` — dumps the active
theme's live state (or another registered theme's on-disk JSON) to an
arbitrary file
- `importThemeFromFile(filePath, *err)` — validates schema, picks a
destination name from the JSON's `"name"` field (file stem fallback),
sanitises against path traversal, refuses to clobber built-ins, suffixes
`(2)` / `(3)`… on name collisions, copies into
`~/.config/AetherSDR/themes/`, makes it active. Returns the final name
on success.

## Theme Editor changes

- **"Theme actions" menu** gains an Export… / Import… pair below the
existing Rename / Delete actions
- **Drag-and-drop**: drop one or more `.aethertheme` (or `.json` theme)
files anywhere on the dialog window → each imports + activates in order
- Drop validation refuses non-local URLs and any file whose extension
isn't `aethertheme` / `json` — arbitrary files get the no-go cursor at
drag-enter
- Cosmetic: removed the manual `▾` glyph from "Theme actions" button
label (QPushButton with `setMenu()` auto-renders one; the two collided)

## File format (`.aethertheme`)

Plain JSON, identical shape to the user-dir `themes/*.json` files (same
`schemaVersion`, `name`, `author`, `version`, `description`, `tokens`).
Sharing happens via file exchange — Discord, Discussions, gist, email
attachment. No hosted marketplace.

## Format compatibility

- `schemaVersion = 1` is current; `> 1` loads with a log warning instead
of refusing — unknown tokens round-trip unchanged so a future v2 theme
survives a v1 build
- Missing tokens on import fall back to built-in defaults via the
existing `seedBuiltinDefaults()` path

## Test plan

- [x] Builds clean (`--target all`)
- [x] Save As "My Glassy Plasma" → Theme actions → Export to file… →
save as `.aethertheme`
- [x] Switch to Default Dark → Theme actions → Import from file… →
re-imports + activates
- [x] Drag the same file onto the dialog → re-imports as "My Glassy
Plasma (2)"
- [x] Drop a non-theme file → silently refused at drag-enter
- [x] Single Qt-rendered dropdown arrow on the Theme actions button (was
two)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>