feat(i18n): add internationalization with DE, FR, ES, PT translations (#12)

Add a compile-time i18n system with 270 translation keys across 5 locales
(EN, DE, FR, ES, PT). Translations are embedded via include_str! and parsed
lazily into flat HashMaps with English fallback for missing keys.

- Add src/i18n module with Locale enum, t()/tw() lookup functions, and tests
- Add JSON translation files for all 5 locales under assets/i18n/
- Provide locale Signal via Dioxus context in App, persisted to localStorage
- Replace all hardcoded UI strings across 33 component/page files
- Add compact locale picker (globe icon + ISO alpha-2 code) in sidebar header
- Add click-outside backdrop dismissal for locale dropdown

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Co-authored-by: Sharang Parnerkar <parnerkarsharang@gmail.com>
Reviewed-on: #12

Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # Stage 1: Generate dependency recipe for caching
 FROM rust:1.89-bookworm AS chef
 RUN cargo install cargo-chef
@@ -15,16 +16,26 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     pkg-config libssl-dev curl unzip \
     && rm -rf /var/lib/apt/lists/*
 
+# Install sccache for compile caching across Docker builds
+RUN curl -fsSL https://github.com/mozilla/sccache/releases/download/v0.9.1/sccache-v0.9.1-x86_64-unknown-linux-musl.tar.gz \
+    | tar xz --strip-components=1 -C /usr/local/bin/ sccache-v0.9.1-x86_64-unknown-linux-musl/sccache \
+    && chmod +x /usr/local/bin/sccache
+
+ENV RUSTC_WRAPPER=/usr/local/bin/sccache
+ENV SCCACHE_DIR=/tmp/sccache
+
 # Install bun (for Tailwind CSS build step)
 RUN curl -fsSL https://bun.sh/install | bash
 ENV PATH="/root/.bun/bin:$PATH"
 
 # Install dx CLI from source (binstall binaries require GLIBC >= 2.38)
-RUN cargo install dioxus-cli@0.7.3 --locked
+RUN --mount=type=cache,target=/tmp/sccache \
+    cargo install dioxus-cli@0.7.3 --locked
 
 # Cook dependencies from recipe (cached layer)
 COPY --from=planner /app/recipe.json recipe.json
-RUN cargo chef cook --release --recipe-path recipe.json
+RUN --mount=type=cache,target=/tmp/sccache \
+    cargo chef cook --release --recipe-path recipe.json
 
 # Copy source and build
 COPY . .
@@ -33,7 +44,8 @@ COPY . .
 RUN bun install --frozen-lockfile
 
 # Bundle the fullstack application
-RUN dx bundle --release --fullstack
+RUN --mount=type=cache,target=/tmp/sccache \
+    dx bundle --release --fullstack
 
 # Stage 3: Minimal runtime image
 FROM debian:bookworm-slim AS runtime