feat(rime): share the Rime config and user dictionary across Linux and macOS

Move the Rime data, preferences, and user dictionary into home/base/gui/rime so fcitx5-rime and Squirrel read the same files.

home/base/gui/rime/README.md

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+# Rime (flypy) customisation
+
+Personal Rime setup for the [flypy](https://flypy.com/) (小鹤音形) schema, shared by fcitx5-rime
+(Linux) and Squirrel (macOS) so both behave the same.
+
+## Files
+
+- `flypy.custom.yaml`: the preferences, layered by Rime on top of the shared rime-data.
+- `flypy_user.txt`: the user dictionary (custom phrases, symbols, missing characters).
+- `default.nix`: per-platform wiring (see that file for the details).
+
+The schema data comes from the `rime-data-flypy` package in `nur-ryan4yin` and stays untouched, so
+updates survive. Personal preferences and the dictionary live here. On macOS the shared data
+includes an empty `flypy_user.txt`, so the user files set `home.fileOverlapResolution = "override"`
+to win over it.
+
+## Preferences
+
+- Right Shift switches CN/EN. Left Shift stays a plain modifier; Ctrl+Space also switches (fcitx5 on
+  Linux, the system input-source shortcut on macOS).
+- Shift+Space does not toggle full/half width.
+- `[ ] { }` produce corner brackets `「」『』`.
+
+These sit in a schema-level patch rather than `default.custom.yaml` so they also override the
+`default` preset and flypy's own bindings, and to avoid colliding with the `default.custom.yaml`
+that ships in Squirrel's user directory.
+
+## Hotkeys
+
+The flypy schema brings its own bindings. Two worth remembering:
+
+- Ctrl+J toggles simplified/traditional Chinese (简繁).
+- Ctrl+. toggles Chinese/ASCII punctuation (中英标点).
+
+## Applying changes
+
+`flypy.custom.yaml` is a store symlink with an epoch mtime, so Rime does not redeploy when it
+changes. Remove `build/` and reload:
+
+```sh
+# Linux
+rm -rf ~/.local/share/fcitx5/rime/build
+fcitx5-remote -r
+
+# macOS: remove build/, then restart Squirrel
+rm -rf ~/Library/Rime/build
+```