Add downloadable/re-uploadable custom themes (ticket 033)

Themes download as a small `:root { --c-*: ...; }` CSS file (full
canonical variable set); uploading validates that set is complete and
registers the file as a CssEntry the theme picker can select. A new
`is_theme` flag on CssEntry distinguishes theme entries from arbitrary
imported/linked CSS so they don't leak into the page unconditionally
like the existing "Configured CSS" list does — a selected custom
theme's CSS is injected only while active, with built-in html.<scheme>
classes removed so its :root override isn't outranked in the cascade.
Linked (URL) custom themes reuse the existing CSS link mechanism and
inherit its non-deletable semantics.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/dto.rs

@@ -136,18 +136,25 @@ pub struct ErrorDto {
 }
 
 /// Request body for `POST /api/settings/css`: raw CSS content to import and
-/// store alongside the settings file.
+/// store alongside the settings file. `is_theme` (ticket 033) marks this as
+/// a selectable colour-scheme theme rather than arbitrary CSS; defaults to
+/// `false` (arbitrary CSS) when omitted.
 #[derive(Debug, Deserialize)]
 pub struct ImportCssRequest {
     pub filename: String,
     pub content: String,
+    #[serde(default)]
+    pub is_theme: bool,
 }
 
 /// Request body for `POST /api/settings/css/link`: a reference to an
-/// external CSS path/URL that ottyl doesn't copy or own.
+/// external CSS path/URL that ottyl doesn't copy or own. `is_theme` (ticket
+/// 033) marks this as a selectable theme rather than arbitrary CSS.
 #[derive(Debug, Deserialize)]
 pub struct LinkCssRequest {
     pub path_or_url: String,
+    #[serde(default)]
+    pub is_theme: bool,
 }
 
 /// Response shape for a single CSS entry, used both by the list endpoint and
@@ -156,12 +163,15 @@ pub struct LinkCssRequest {
 /// linked path/URL for `Linked` entries (see `settings::css_entry_id`) —
 /// this lets the frontend key/reference both kinds of entries uniformly,
 /// even though only `Imported` entries are deletable/content-fetchable.
+/// `is_theme` (ticket 033) distinguishes a theme entry (selectable from the
+/// theme picker) from arbitrary imported/linked CSS.
 #[derive(Debug, Clone, Serialize)]
 pub struct CssEntryDto {
     pub id: String,
     pub kind: CssEntryKind,
     pub filename: Option<String>,
     pub path_or_url: Option<String>,
+    pub is_theme: bool,
 }
 
 #[derive(Debug, Clone, Copy, Serialize)]
@@ -174,17 +184,19 @@ pub enum CssEntryKind {
 impl From<&CssEntry> for CssEntryDto {
     fn from(entry: &CssEntry) -> Self {
         match entry {
-            CssEntry::Imported { id, filename } => CssEntryDto {
+            CssEntry::Imported { id, filename, is_theme } => CssEntryDto {
                 id: id.clone(),
                 kind: CssEntryKind::Imported,
                 filename: Some(filename.clone()),
                 path_or_url: None,
+                is_theme: *is_theme,
             },
-            CssEntry::Linked { path_or_url } => CssEntryDto {
+            CssEntry::Linked { path_or_url, is_theme } => CssEntryDto {
                 id: crate::settings::css_entry_id(entry),
                 kind: CssEntryKind::Linked,
                 filename: None,
                 path_or_url: Some(path_or_url.clone()),
+                is_theme: *is_theme,
             },
         }
     }
@@ -201,6 +213,9 @@ pub struct SettingsDto {
     pub active_profile: Option<String>,
     /// `None` means unlimited (ticket 026).
     pub max_logs: Option<usize>,
+    /// Id of the selected custom theme entry, if any (ticket 033). `None`
+    /// means `color_scheme` alone determines the active palette.
+    pub custom_theme_id: Option<String>,
 }
 
 impl From<&Settings> for SettingsDto {
@@ -209,20 +224,23 @@ impl From<&Settings> for SettingsDto {
             color_scheme: settings.color_scheme,
             active_profile: settings.profiles.active_profile.clone(),
             max_logs: settings.max_logs,
+            custom_theme_id: settings.custom_theme_id.clone(),
         }
     }
 }
 
 /// Request body for `PATCH /api/settings`. `color_scheme`: `None` leaves the
-/// current value untouched. `max_logs`: distinguishes "field omitted" (leave
-/// unchanged, outer `None`) from "field present" (inner `Option<usize>`
-/// applied verbatim — `null` clears the cap back to unlimited, a number sets
-/// it), via `deserialize_double_option`.
+/// current value untouched. `max_logs`/`custom_theme_id`: each distinguishes
+/// "field omitted" (leave unchanged, outer `None`) from "field present"
+/// (inner `Option` applied verbatim — `null` clears it, a value sets it), via
+/// `deserialize_double_option`.
 #[derive(Debug, Deserialize)]
 pub struct UpdateSettingsRequest {
     pub color_scheme: Option<ColorScheme>,
     #[serde(default, deserialize_with = "deserialize_double_option")]
     pub max_logs: Option<Option<usize>>,
+    #[serde(default, deserialize_with = "deserialize_double_option")]
+    pub custom_theme_id: Option<Option<String>>,
 }
 
 /// Distinguishes a missing JSON field (outer `None`, via `#[serde(default)]`