feat: add theme_mode, theme_light, theme_dark to UserAppearanceSettings (#25076)

Part 1: Backend portion of a change broken into 2 PRs.
Part 2: #25077 

Adds three new UserAppearanceSettings fields (theme_mode, theme_light,
theme_dark) on top of the existing theme_preference and terminal_font.
Replaces GetUserThemePreference and GetUserTerminalFont with a single
GetUserAppearanceSettings aggregate query. The PUT handler is wrapped in
db.InTx so sync-mode's mode + slot writes can never half-apply.

docs/reference/api/users.md

@@ -547,6 +547,9 @@ curl -X GET http://coder-server:8080/api/v2/users/{user}/appearance \
 ```json
 {
   "terminal_font": "",
+  "theme_dark": "string",
+  "theme_light": "string",
+  "theme_mode": "",
   "theme_preference": "string"
 }
 ```
@@ -578,6 +581,9 @@ curl -X PUT http://coder-server:8080/api/v2/users/{user}/appearance \
 ```json
 {
   "terminal_font": "",
+  "theme_dark": "light",
+  "theme_light": "light",
+  "theme_mode": "sync",
   "theme_preference": "string"
 }
 ```
@@ -596,6 +602,9 @@ curl -X PUT http://coder-server:8080/api/v2/users/{user}/appearance \
 ```json
 {
   "terminal_font": "",
+  "theme_dark": "string",
+  "theme_light": "string",
+  "theme_mode": "",
   "theme_preference": "string"
 }
 ```