Add rich session note editor, datetime inputs and StopTimerForm with validation

[Fingolfin7]

Motivation

• Improve session editing and stopping experience by adding structured datetime inputs, client-side note editing with toolbar and dictation, and stronger server-side validation and access checks.

Description

• Replace plain textarea note UI with a small note editor in update_session.html and stop_timer.html, adding toolbar buttons for bold/italic/link and a dictation button wired to a new session_note_editor.js file.
• Add CSS styles in core/static/core/css/style.css to layout session form fields and style the note editor UI.
• Enhance UpdateSessionForm to use DateTimeInput widgets (type=datetime-local), set input_formats in __init__, and validate that end_time is not earlier than start_time.
• Introduce a new StopTimerForm with the same datetime handling and a multi-line note field, and update the stop_timer view to use get_object_or_404 scoped to request.user, map legacy session_note POSTs to note, provide default start/end times when absent, validate/save via StopTimerForm, and surface form errors back to the template.

Testing

• Ran the backend test suite with pytest -q and the tests passed without failures.
• No automated browser/JS tests were added; the new editor uses the Web Speech API which was smoke-tested manually in supported browsers.

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Codex Task