refactor: consolidate bulk actions, URL assembly, and case-artefact filenames

Pulls three pattern groups into shared helpers so a rename or wire-format
tweak edits one file instead of 10+:

- BulkAction enum in doctate-common replaces the "close"/"analyze"/"reset"
  string literals that were duplicated between the bulk handler and 3
  attack/CSRF test files. FromStr preserves the exact "Unbekannte Aktion"
  error shape; the handler match is now exhaustive over the enum.
- join_url helper in doctate-common absorbs 7 identical
  trim_end_matches+format! call sites across client-core, client-desktop,
  server/transcribe (ollama, whisper), and server/analyze (llm).
- server/tests/common/artefacts.rs re-exports ONELINER_FILENAME
  (doctate-common), DOCUMENT_FILE + ANALYSIS_INPUT_FILE
  (doctate-server::analyze), and CLOSE_MARKER (doctate-server::paths) so
  test files reference the canonical name instead of inlining literals.

Also lifts client-desktop local duplication:
- paths.rs: project_path helper collapses 4 identical ProjectDirs chains
- main.rs: or_die helper replaces 4 eprintln!+exit(1) blocks
- app.rs: named RecordingContext struct replaces (Uuid, String) tuple
  at 3 sites around the ffmpeg-flush finalization path

Verification: 397 tests pass (baseline was 389; +8 for new unit tests on
BulkAction and join_url), 0 failed, 4 ignored (unchanged). clippy clean.

server/src/transcribe/ollama.rs

@@ -1,5 +1,6 @@
 use std::time::Duration;
 
+use doctate_common::join_url;
 use serde::{Deserialize, Serialize};
 use tracing::debug;
 
@@ -100,7 +101,7 @@ pub async fn generate_oneliner(
     transcript: &str,
     timeout: Duration,
 ) -> Result<String, OllamaError> {
-    let url = format!("{}/api/chat", ollama_url.trim_end_matches('/'));
+    let url = join_url(ollama_url, "/api/chat");
     debug!(%url, model, "generating oneliner");
 
     let body = ChatRequest {

server/src/transcribe/whisper.rs

@@ -1,6 +1,7 @@
 use std::path::Path;
 use std::time::Duration;
 
+use doctate_common::join_url;
 use reqwest::multipart::{Form, Part};
 use tracing::debug;
 
@@ -100,9 +101,8 @@ pub async fn transcribe(
         .filter(|s| !s.is_empty())
         .unwrap_or("de");
     let url = format!(
-        "{}/asr?output=txt&language={}",
-        whisper_url.trim_end_matches('/'),
-        language
+        "{}?output=txt&language={language}",
+        join_url(whisper_url, "/asr")
     );
     debug!(%url, "posting to whisper");