feat(tasks): add activity log, Markdown editor, and dark-mode polish

Backend
- Add TaskActivity model to record start/pause/review/complete/cancel/reopen
- Preserve started_at when reopening tasks; keep timestamps in app-local time
- Expose recent activities on task detail view

Migrations
- Add 004_add_task_activities_table (FKs + indexes)

Task detail (UI)
- Move Description into its own card and render as Markdown (markdown + bleach)
- Restyle Quick Actions to use dashboard btn-action styles
- Add custom confirmation modal + tooltips
- Recent Time Entries: use dashboard action buttons, add progressive “Show more/less” (10 per click)

Tasks list
- Fix dark mode for filter UI (inputs, selects, input-group-text, checkboxes)

Create/Edit task
- Integrate EasyMDE Markdown editor with toolbar
- Strong dark-theme overrides (toolbar, editor, preview, status bar, tokens)
- Prevent unintended side-by-side persistence
- Align “Current Task Info” dark-mode styles with task detail

CSS
- Add dark-mode tints for action buttons, tooltip light theme in dark mode
- Editor layout polish (padding, focus ring, gutters, selection)
- Quick actions layout: compact horizontal group

Deps
- Add: markdown, bleach

Run
- flask db upgrade  # applies 004_add_task_activities_table

app/utils/template_filters.py

@@ -1,5 +1,11 @@
 from flask import Blueprint
 from app.utils.timezone import utc_to_local, format_local_datetime
+try:
+    import markdown as _md
+    import bleach
+except Exception:
+    _md = None
+    bleach = None
 
 def register_template_filters(app):
     """Register custom template filters for the application"""
@@ -38,3 +44,27 @@ def register_template_filters(app):
         if text is None:
             return ""
         return text.replace('\n', '<br>')
+
+    @app.template_filter('markdown')
+    def markdown_filter(text):
+        """Render markdown to safe HTML using bleach sanitation."""
+        if not text:
+            return ""
+        if _md is None:
+            # Fallback: escape and basic nl2br
+            try:
+                from markupsafe import escape
+            except Exception:
+                return text
+            return escape(text).replace('\n', '<br>')
+
+        html = _md.markdown(text, extensions=['extra', 'sane_lists', 'smarty'])
+        if bleach is None:
+            return html
+        allowed_tags = bleach.sanitizer.ALLOWED_TAGS.union({'p','pre','code','img','h1','h2','h3','h4','h5','h6','table','thead','tbody','tr','th','td','hr','br','ul','ol','li','strong','em','blockquote','a'})
+        allowed_attrs = {
+            **bleach.sanitizer.ALLOWED_ATTRIBUTES,
+            'a': ['href', 'title', 'rel', 'target'],
+            'img': ['src', 'alt', 'title'],
+        }
+        return bleach.clean(html, tags=allowed_tags, attributes=allowed_attrs, strip=True)