Improve analysis UX: algorithm explanations, skeleton loading, auto-expand achieved specs

- Replace terse one-line optimization mode descriptions with clearer multi-sentence
  explanations of how Maximize Count and Priority Order algorithms behave
- Add skeleton loading placeholders on course buttons while analysis is pending
- Auto-expand achieved specializations to show credit breakdown by default
- Add instructional subtitles to Course Selection and Specializations sections
- Make Clear and Clear All buttons more prominent with visible backgrounds

openspec/changes/analysis-ux-improvements/specs/analysis-skeleton-loading/spec.md

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+## ADDED Requirements
+
+### Requirement: Skeleton placeholder during analysis
+When an elective set is not pinned and analysis is pending, the CourseSelection component SHALL display skeleton placeholder elements in place of the course choice buttons.
+
+The skeleton placeholders SHALL:
+- Visually approximate the size and layout of the course choice buttons they replace.
+- Display a pulsing/animated background to indicate loading.
+- Replace the current "analyzing..." text label.
+
+#### Scenario: Analysis pending for an unpinned set
+- **WHEN** an elective set is not pinned AND `loading` is true AND no analysis result exists for that set
+- **THEN** skeleton placeholder rectangles SHALL be displayed instead of course choice buttons
+
+#### Scenario: Analysis completes for a set
+- **WHEN** an analysis result arrives for a previously loading set
+- **THEN** the skeleton placeholders SHALL be replaced with the actual course choice buttons showing analysis data
+
+#### Scenario: Set is already pinned
+- **WHEN** an elective set has a pinned course
+- **THEN** no skeleton placeholders SHALL be shown regardless of loading state